Formatted Title
Unlocking the Power of Natural Attenuation: Insights from Laboratory Studies and Field Sampling Approaches
Background/Objectives
Laboratory treatability studies and field sample collections are commonly used for proof-of-concept and to determine site-specific remediation design parameters. These methods have been used to develop groundwater remedial options for a wide variety of contaminants including those found at coal combustion residual (CCR), industrial groundwater, manufacturing and former mine sites. An important consideration in groundwater remedy selection is the suitability of monitored natural attenuation (MNA) as a remedy component or as a stand-alone passive remedy. Part of evaluating whether MNA is an appropriate remedial technology at a site involves the demonstration of the type(s) and longevity of ongoing natural attenuation processes, including the capacity of an aquifer to attenuate site-specific constituents. This requires the characterization of groundwater and/or aquifer solids geochemistry as well as subsequent column and/or leachability testing.
Approach/Activities
This presentation will focus on the use of (i) a suite of field sample collection tools and data analysis approaches and (ii) batch reactor and column treatability testing to evaluate groundwater treatment options for redox sensitive metals in CCR-impacted groundwater, including enhanced and natural attenuation options.
In two separate demonstrations, a comprehensive laboratory program was developed and performed based upon the USEPA’s tiered approach to identify the natural attenuation processes, rates, attenuation capacities, and longevity of 1) arsenic impacted groundwater or 2) molybdenum (Mo), sulfate (SO42-), and boron (B) impacted groundwater from two different CCR impoundment sites. The programs included detailed evaluation of the aqueous geochemistry, aquifer matrix minerology and chemical composition, As speciation, adsorption and desorption reactions, and stability of the immobilized constituents.
Results/Lessons Learned
The laboratory results, along with detailed conceptual site models, were used to develop hydrogeochemical site models that described the fate of i) dissolved As and ii) dissolved Mo, SO42-, and B downgradient of two separate CCR impoundments. At the CCR impoundment impacted with As the data showed that the aquifer solids were able to attenuate the As in a reasonable time supporting MNA as a potential field remedy. Similarly, for the second CCR impoundment site the laboratory testing was used to develop site-specific sorption coefficients for Mo, SO42- and B. Results additionally indicated that after sorption to the site aquifer materials, potential desorption of the analytes back into groundwater would not be affected by redox conditions and was not associated with iron in the aquifer material, providing insight into the mechanisms of attenuation. These findings helped to support long-term monitoring as part of the corrective action plan submitted to the regulator. This presentation will describe the field sampling and laboratory testing used to generate the data required to develop the site models. Results will be presented with descriptions of how the data are used to support the MNA remedy in the field. Lessons learned and improvements for future studies will also be discussed.